Following their application, coating materials are required to display great steadfastness, so as to allow the production of comparatively thick coating films and coating systems, without this leading to extremely disruptive running, especially on vertical substrates.
“Running” is the term for the sagging of applied coating materials on vertical or inclined surfaces, producing an unattractive appearance in the resultant coatings. Where this run phenomenon occurs across a relatively large area, it is also called “curtaining”. In general a distinction is made between runs at edges, angles, and holes (initiator points) and the extensive sagging of coatings on surfaces, which is also called “slipping”. The reason for the formation of runs may lie in an incorrect composition or in incorrect application of coating material. The quantity indicated as the “run limit” is generally the dry film thickness of the applied coating material, in μm, above which the first runs occur following spray application of said material to a perforated, vertical metal panel (cf. in this respect also Römpp-Online 2002, “running”, “run limit”, and “curtaining”). The dry film thickness or dry coat thickness of the applied coating material above which the running phenomena described occur on vertical or inclined surfaces is generally referred to as the “stability limit”.
In practice these running phenomena constitute a serious problem, since in the industrial coating of three-dimensional substrates of complex shape, especially in the case of automotive OEM finishing, they lower operational reliability and raise the reject rate. For instance, in connection with the finishing of automobile bodies, there is a risk that the films built up in electrostatic spay application (ESTA) on sharp edges of the bodies will be too thick. If their thickness exceeds the stability limit of the coating material in question, the disruptive running phenomena will occur in the course of further processing, particularly during drying and thermal curing. On the other hand, the viscosity of the coating materials must not be so high that problems occur during application and it is no longer possible for the applied coating films to flow out effectively.
These problems occur to an increased extent with coating materials, especially clearcoat materials, having a high solids content, which are also referred to by those in the art as “high-solids clearcoat materials”. The use of coating materials of high solids content, especially high-solids clearcoat materials, is advantageous on environmental grounds, though, since their emissions of volatile organic materials in the course of application in curing are lower. At the same time these coating materials as well must provide coating systems, especially clearcoats, which in terms of their gloss, transparency and clarity, scratch resistance, weathering stability, and yellowing resistance satisfy all of the demands of the market.
This problematic behavior in these coating materials, especially in the high-solids clearcoat materials, arises from the fact that they are required, despite having a low solvent content, to be of low viscosity, in order to be able to be applied easily by spray application. This implies, however, that, unlike coating materials having higher solvent contents, the increase in viscosity occurring as a result of evaporation during spray application can only be small. This makes it fundamentally necessary to provide them with rheological aids.
Comparable problems also occur with adhesives and sealants, especially those having a high solids content.
Rheological aids for setting a pseudoplastic behavior (cf. Römpp Lexikon Lacke und Druckfarben, Georg Thieme Verlag, Stuttgart, New York, 1998, “Pseudoplasticity”, page 546) and pseudoplastic coating materials comprising them have been known for a long time. In particular the rheology of coating materials can be positively influenced using liquid rheological aids comprising crystalline urea derivatives. These rheological aids, often also referred to as thixotropic agents, are prepared, familiarly, in situ from polyisocyanates and amines in the presence of additives. By way of example, reference may be made in this respect to the German published laid-open specifications and patent applications and to the German patent DE 23 60 019 B 2, DE 23 59 9123 B 1, DE 23 59 129 B 1, DE 198 11 471 A 1, DE 27 51 761 C 2, DE 199 24 170 A 1, DE 199 24 172A 1, DE 199 24 171 A 1, DE 100 42 152A 1, DE 101 26 647A 1 or DE 101 26 648 A 1, to European patent EP 0 192 304 B 1 or to international patent applications WO 94/22968 A 1 and WO 00/37520 A 1.
These known, liquid rheological aids, however, contain the crystalline urea derivatives only in an amount of up to 10% by weight, based on a rheological aid. This means, though, that a comparatively large amount of liquid rheological aid must be incorporated into the coating materials, particularly into the high-solids clearcoat materials, in order to obtain an amount of crystalline urea derivatives sufficient for positively influencing the rheology. As a result of the comparatively large amount of liquid rheological aid, however, the solids content of the coating materials is undesirably lowered.
Attempts have been made to counter this problem by raising the amount of crystalline urea derivatives in the known, liquid rheological aids. In the majority of cases, however, this results in the liquid rheological aids in question being barely still fluid and therefore subsequently being difficult if not impossible to process.
It is an object of the invention to provide novel liquid rheological aids which comprise at least one urea derivative and at least one additive and which no longer have the disadvantages of the prior art but instead, even with a urea derivatives content of more than 10% by weight, based on the novel liquid rheological aid, are easy to prepare, are fluid, and are easy to process.
The novel liquid rheological aids ought to allow greater amounts of coating materials, adhesives and sealants, especially high-solids clearcoat materials, to be treated than with the same amount of existing liquid rheological aids.
The novel coating materials, adhesives, and sealants, especially the novel high-solids clearcoat materials, treated with the novel liquid rheological aids ought to have a particularly high storage stability. Their steadfastness should be particularly high, so that they can be applied without problems at high film thicknesses without any running.
The novel coating materials ought to provide coatings, adhesive layers, and seals having outstanding performance properties. In particular the novel high-solids clearcoat materials ought to provide clearcoats having particularly high film thicknesses which exhibit outstanding leveling, are free from surface defects, such as gel specks, runs, pin holes, orange peel, stress cracks (mudcracking) or craters, and have an outstanding overall appearance, high scratch resistance, high weathering stability, and high condensation resistance.